JC virus (JCV) is the etiologic agent of progressive multifocal leukoencephalopathy (PML). Epidemiologic studies reveal that there is a high prevalency and a poorly understood latency associated with this polyomavirus. Recent data indicates that the central nervous system may be a repository of latent JCV. JCV, like other polyomaviruses, can induce neoplasms in experimental animals and transform tissue culture cells, however JCV is the only polyomavirus capable of inducing tumors in non- human primates. The high prevalence and latency of the virus, the increasing incidence of and inadequate therapy for PML, the possibility of an association with a subset of human tumors, and an encoded oncogene whose function is poorly understood makes the study of the biology of JCV important. This part of the application contributes to the program project through providing a systematic and reliable mechanism for the pathologic analysis of experimental animals, tumor specimens, and development of cell cultures. Integrating the pathologic expertise of this core with the other projects will ensure that analysis, handling, and storage of specimens will be uniform, consistent, and efficient.